Go to the Options menu and select Preferences. Or press the keyboard shortcut Ctrl + P.
Winamp Pro is the top page in the Winamp Preferences tree in Winamp 5.66 and earlier.
Registered Name: Enter your Registered Name in the top field. This must be the exact same name you used on the registration form.
Registration Key: Copy and paste the exact Registration Key into the bottom field.
Submit Registration Key: Click to register. Say 'Yes' to any UAC Prompt (Windows Vista/7/8), if enabled. Note, turns into a 'Remove Registration Key' button after successful registration.
If it says "Invalid registration key", make sure you are using the exact correct Name & Key and there are no trailing spaces. In some cases, you might need to run Winamp in admin mode (e.g. by right-clicking the Winamp desktop icon and selecting 'Run as administrator') for the key to be accepted.
Q: The "Winamp Pro" preferences page is missing. I don't see it at the top of the Prefs tree.
A: The only way this can happen is if you've installed the Lite'N Winamp Preferences plug-in (gen_nopro.dll) from the Winamp Essentials Pack. Go to: Prefs > Plug-ins > General Purpose > gen_nopro.dll > Configure, and uncheck "Winamp Pro" from the "Hide" menu.
Also note that the Winamp Pro page no longer exists as from Winamp 5.8 due to Winamp now being 100% freeware again.